Recent developments in container-molding techniques have tended toward the use of extremely light-weight containers for packaging etc., for the obvious reasons of material saving and faster molding cycles. As containers become lighter in weight, the wall thickness of course diminishes.
By way of example, a "thin-wall" molded 500 g margarine container weighs 13.5 g and has a wall thickness of 0.45 mm. Such a container has a molding cycle of 3.2 seconds.
While it would be possible to reduce the wall thickness still further in order to produce even lighter parts, typically by selecting so-called easy-flowing molding materials such as polyethylene or polypropylene, a problem arises with extremely thin-walled containers in regard to the ejection of the molded workpieces from the core.
The conventional method for ejecting cup-shaped workpieces with closed front ends and rearwardly facing rims uses stripper rings, as shown in my prior U.S. Pat. No. 4,179,254, owned by the assignee of my present application, supplemented by additional features such as venting holes, slots or air valves in the mold core to break the vacuum created between the core and the molded part or workpiece during ejection, and to prevent collapsing of the workpiece at that stage.
The use of stripper rings to eject the container by pressing on its rim requires that the container be reasonably stiff so that it can be stripped off the core without buckling or even "folding over itself" like the finger of a tight-fitting glove during removal.
However, the demand for such stiffness to facilitate removal of the container from the core implies greater wall thickness, stiffer rather than easy-flowing materials, and longer cooling time. Any one of these factors contributes to slower cycles and higher costs.
An obvious answer to this difficulty, and one often attempted, has been to increase the air supply through the air vents or valves within the core designed to "blow" the workpieces off the core. This is sometimes successful especially with shallow workpieces and with strongly tapered ones that have a significant difference between top and bottom diameters. However, this method has not been satisfactory for deeper containers, particularly when their peripheral walls are nearly cylindrical, i.e. with a very small draft angle. In such cases, the air pressure inside the container tends to stretch its closed front end or bottom and thereby tighten the grip of the container on the core. The plastic may even burst at the cup bottom.
Another problem arising with this method is the so-called Venturi effect caused by the air escaping at the cup bottom and blowing out through the gap between the core and the plastic workpiece. At a certain point, the Venturi effect creates a suction acting on the workpiece so that it will advance only a short distance and then "hang" in midair on the core without falling free. Occasionally this difficulty can be overcome by a large increase in airflow, but this expedient is usually confined to single-cavity molds. In multi-cavity molds, unless an extraordinarily large air supply is introduced, the first core to clear lets most of the air escape while the other cores are "starved" and fail to eject the workpieces. Also, the use of large volumes of high-pressure air is wasteful. Furthermore, such air ducting and valving near the top of the core presents severe restrictions in the layout of effective cooling channels near the tip of the core where cooling is most needed, thus resulting in poorer cooling and slower cycles.
One method already in practice for overcoming some of the deficiencies of the vents or valves in the top of the core (adjacent the bottom of the container) is to split the core at approximately 2/3 to 7/8 of its height and to make the tip of the core as a separate piece, either from mold steel or from such better heat-conducting materials as beryllium-copper. The seat of this core tip is so constructed that either a multitude of radial slots or a single continuous slot is created. By providing the slot with a width of approximately 0.015 mm, the slot becomes narrow enough to prevent plastic from flowing into it, but sufficient to let air under pressure pass through when required. The slots are connected by channels to the main air supply adjacent the core, and the channels are so located that they do not interfere with optimal layout for the cooling channels.
This blow-off directed onto the peripheral wall of the container (rather than its closed end) overcomes the problem of vents or valves on the top of the front face or core, by stretching the container away from the periphery of the core and loosening it even with a very small draft angle. However, this method does not solve the problem created by the Venturi effect. The workpieces move partly off the core and then "hang" as described earlier.